Photo-induced chemical vapour deposition of silicon oxide thin films

170

Thin Solid Films, 218 (1992) 170 181

Photo-induced chemical vapour deposition of silicon oxide thin films P. G o n z g , lez, D. F e r n f i n d e z , J. P o u , E. G a r c i a , J. S e r r a , B. L e 6 n a n d M . P 6 r e z - A m o r Departnwnt ~?] Applied Physics. Uniz;ersit3 ~]' Vigo, P.O. Box 62, 36200 Vigo (Spa#7)

Abstract

A review of the photo-induced chemical vapour deposition (photo-CVD) processes yielding silicon oxide thin films that have emerged in the last decade is presented. Both lasers and UV lamps as photon sources are included. The basic principles, processing conditions, precursors, geometries, advantages and limitations of the various types of photo-CVD processes are described and compared. Their technological applicability and potential for industrial large-scale installation are discussed.

1. Introduction

In the last decade, considerable interest has emerged in photo-induced chemical vapour deposition (photoCVD) techniques for depositing silicon dioxide owing to the advantages that these methods offer compared with other conventional deposition techniques. First, the substrate temperature during deposition can be greatly reduced because high energy photons induce the chemical reaction. In this way, some disadvantages encountered in thermal oxidation, or thermal CVD, such as wafer warpage, dopant redistribution and defect generation, are avoided. Second, other low temperature techniques such as atmospheric CVD or low pressure CVD, while reducing process temperatures, are deficient in uniformity, purity and film stability. On the contrary, although the plasma-enhanced CVD technique has made low temperature deposition possible with improved film properties, in this case the substrate is bombarded with energetic particles that may lead to physical and chemical damage to the substrate, the substrate-layer interface and/or the growing film. Another disadvantage of this method is that the plasma potential is more positive than the chamber walls, causing ion acceleration that enhances impurity sputtering from the walls and film quality degradation. Finally, the process parameters, namely electrode spacing, r.f. power and frequency, total pressure and gas flow, are so strongly interrelated that it is very difficult to control the effect of one single parameter and to optimize the process. If account is taken of these considerations, the photochemical deposition process is a very promising technique for the production of insulating thin films, since the reaction energy is selectively provided by photons and, moreover, the deposition area can be selected by

0040-6090/92/$5.00

locally irradiating the substrate. Photo-CVD allows more independent control of the processing parameters and by tuning a single parameter it is possible to modify the film characteristics. This ensures an independent optimization of the process variables and a broader process window compared with conventional deposition methods. Many technological fields can profit from the advantages of photo-CVD processes. Some of the very promising industrial applications of photo-CVD silica films are direct writing deposition for circuit repair and for microwave and high speed digital circuits, final passivation and intermediate insulator layers in multilevel devices, trilevel resists for high resolution lithography, protective coatings against corrosion at high temperatures, and direct patterning by optical projection for producing microlenses, diffraction gratings and optical waveguides. In this paper, a review of all photo-CVD processes leading to the growth of silicon oxide thin films is presented. The photo-CVD processes discussed include both lasers and UV lamps as photon sources. The principles, reactions involved, experimental details, advantages, processing parameters and possible applications of the various types of photo-CVD processes are described and compared.

2. UV lamps

Lamps are widely applied in photo-CVD processes (Table 1) owing to their low cost and easy handling. For large area deposition or whole wafer processing these photon sources seem more appropriate than lasers. Lamps operating in the vacuum UV (VUV) are used (Fig. 1) most often because high energy photons are required to dissociate the gas precursors.

,~ 1992

ElsevierSequoia. All rights reserved

126 121.5 + !47.0- 185 121.5 + I85

electron

I mbar

100 180

Arf excimer D, + Xe-Hg

0.6 mbar 0.2 Torr

SEM, scanning

microscopy;

25-390

Si, Hs + 0,

spectroscopy;

0.2 Torr

150

30-300

SiH, + NaO S&H, + 0,

“AES, Auger electron spectroscopy: XPS. X-ray photoelectron analysis; ESCA, electron spectroscopy for chemical analysis.

0.2 Torr

Powders 160

25-250 1255350

6 Torr I Torr 0.5 Torr

NOz + SiH,(Ar) SiH, + N,O(N,)

Si, H6 + 0, S&H6 + N,O, SizHs + O2

S&II, + O,(N,)

0-IOTorr

7-20 IO 100 I3 23 120 60 60

50-300 250 84-235 1 IO-310 30-300 25-300 150-350 25-300

SiH4 + N,O SiHJ + O,(N,) SiH, + 0,

Torr

0.3-3

< 400

100-400

SiH, + 0,

0.7-2.0 Torr 0.6-0.7 mbar 1.7 Torr

185 254 I85 254 121.5 121.5 147.0 121.5 121.5 123.6

80 5 13 900 450

80-170

75-175 80 25-300 40-250 350-450 150-350 100-300

80 I50

Total pressure

SiH, + N,O SiHl + NzO N,O + SiH,(H,) Sia H, + N,O N,O + S&H, S&H, + 0, SiH, + O,(Ar)

106.6

D>+Hg

deposition

Growth rate (A min-‘)

vapour

254 253.7 253.1 253.1 253.7 253.7 184.9

Ar windowless N2 windowless

Da Dz Kr

Hg Hg D, Dz Xe

Hg

Hg

sensitization sensitization sensitization sensitization sensitization sensitization

(“C)

Substrate temperature

by lamp chemical

25-200 100-200

Precursors

for the SiO, thin films deposited

SiH, 4 N,O(N,) SiH, + N, 0

Hg Hg Hg Hg Hg Hg

(nm)

Wavelength

conditions

253.1 254

1. Processing

Hg sensitization Hg sensitization

Lamp

TABLE

DLTS,

techniques”

mass

IR, C- V, etch rate

IR, C- V quadrupole

V, ESCA,

spectroscopy;

IR, C

IR, C- V, DLTS

deep level transient

Ellipsometry, etch rate

Ellipsometry,

LIMA,

DLTS,

Ellipsometry, XPS, etch rate, C- V Cathodoluminiscence, SEM, IR Ellipsometry, IR Ellipsometry, Ir, SEM, etch rate IR, DLTS, XPS, AES, etch rate Ellipsometry. C-V, LIMA, IR Ellipsometry, C- V, IR, DLTS, UV spectroscopy Ellipsometry, SEM, C V, IR, etch rate Ellipsometry, LIMA, IR, C- V, etch rate

Ellipsometry, Ellipsometry, spectrometry Ellipsometry,

Ellipsometry, C- V Ellipsometry, C- V, etch rate, IR, AES, adhesion Ellipsometry, adhesion Ellipsometry, IR, etch rate AES, XPS, C- V, etch rate Ellipsometry, C- V, etch rate, AES

Characterization

laser ionization

[28,291

PI t271

~241 v51

[I91 PO1 PII [22,231

(181

mass

t13,141 [l, 7, 15, 16, 171

Cl21

[III

(31 I41 t51 [61 [71 [8-IO]

Ill M

References

172

P. G o n z t ) l e z e t a l . / P h o t o - C V D o / S i o x i d e

AI reflector / ~ ~ U V lamp --~Window

[I

//

Gu exhaust

II

Heater

Substrate

Gas

The rmocouple

I

Inlet

Controller

Fig~ I. Typical experimental system for lamp CVD.

The first process studied for the photodeposition of dielectric thin films was the photosensitization of the gaseous mixture by adding to it trace concentrations of mercury vapour and irradiating with the 253.7 nm line of the mercury lamp. The mercury atoms are resonantly excited by the photon absorption Hg(ground) + hv (2537 ,~)

~ Hg(excited)

and collisions with the reactant molecules cause their dissociation and subsequent chemical reaction. The main disadvantage of this method is the possibility of contamination of the film by mercury. When the precursor gases are Sill 4 and N 2 0 [ 1 - 5 , 30, 31] the dominant primary reaction is the collision of mercury atoms with N 2 0 molecules to generate oxygen atoms, in both the ground and the excited states: Hg* + N 2 0

, N 2 -t- O(ground

Z/

1,9 1.8 .~ 1.7 U z x

x 1'5 I 1,4 0

t 0.05

i

0.1

0.15

SiH4/N20r a t i o

0.2

0,25

Fig. 2. Refractive index vs. SiH4:N20 ratio of films deposited by mercury-sensitized CVD under different conditions: A~ Chen et al. [2]; *, Kim et al. [3]; L S u e t al. [5].

excited) + Hg

Thereafter, the atomic oxygen reacts with silane and intermediate species to yield silica films in a process that is not at present well understood. By varying the SiH4-to-N20 ratio it is possible to deposit films of composition sio,. ( O < x <2), and stoichiometric silica films are deposited for Sill4: N 2 0 ~< 0.05 [2-5] (Fig. 2). The deposition rate (with a typical value of 150/k min -~) and optical properties seem to depend primarily on the UV intensity, reaction chamber geometry and gas pressure, but only weakly on the substrate temperature within the range from 140 to 300 °C [2, 5].

Mercury vapour sensitization has also been used to catalyse the reaction between S i z H 6 and N 2 0 to form SiOx on InSb [6, 7]. With this mixture, as indicated by the refractive index values and the Auger spectra, complete oxidation of the films is not achieved because the N 2 0 is not an oxygen donor capable of efficiently oxidizing disilane molecules [21]. It should be noted that, in addition to N20, disilane is also significantly decomposed by UV radiation of wavelengths shorter than 220 nm. Moreover, some researchers [6, 7] claim that in the S i z H T - N 2 0 system the deposition mechanism may be

173

P. Gonz6lez et al. / Photo-CVD o f Si oxide

explained by a Langmuir-Hinshelwood model, indicating a bimolecular surface reaction process. This fact provides a first idea about how each of the processing parameters affects the process and how complicated a complete explanation of it may be. In order to avoid possible film contamination by the presence of mercury atoms in the gas mixture, great attention was paid to direct photolytic methods using different kinds of lamps and precursor gases. The first attempt was to use the 185 and 254 nm lines of the low pressure mercury lamp for the direct photodissociation of 02 mixed with Si2H 6 or Sill4. The strong absorption of the 185 nm radiation by the oxygen molecules follows the scheme (refs. 13, 27 and 32, p. 180) 02

hv ( 185 n m )

~ 20(3p)

The role of the 254 nm photons in the photodissociation is only auxiliary since they dissociate the ozone molecules produced in a three-body reaction involving atomic and molecular oxygen [ 12, 27]:

O(1D)+N200 2 Jr-

hY ( 185 nm)

'O3+U

hv ( 147 n m )

~ Sill3 + Sill 2 + H

Si2 H6

hr (147 n m )

SizH6

~SiH3SiH + 2H hv ( 147 n m )

~ Si2 H5 + H

These radicals may easily react with other molecules and among themselves, including insertion, decomposition or coupling reactions, such as

2<310nm

O3-

~02 + O(ID)

With this mechanism SiO2 films have been obtained from Sill4 and 02 [ 11, 12, 15-17] at substrate temperatures below 400 °C and high growth rates (40nm min-l). These films exhibit refractive indices between 1.44 and 1.46 and no hydrogen content is observed [ 12]. Takahashi and Tabe [11] have performed more exhaustive studies, observing the existence of two kinetic mechanisms: for low 02 partial pressure the chemical reaction takes place at the substrate surface following the Langmuir-Hinshelwood mechanism and for high pressures the reaction occurs in the gas phase with subsequent deposition on the substrate. This photodeposition process has not yet been well explained and very interesting work is being done by analysing the very early stages of the process through in situ surface characterization [ 15-17, 33]. Similar results have been obtained with a mixture of Si2 H 6 and 02 [8-10] in the temperature range between 150 and 350 °C. By IR spectroscopy it is observed how the film structure improves as the substrate temperature or the UV illumination is increased, yielding better electrical properties. The mixture of Sill4 and N20 was also investigated under illumination by the 185 nm line of a low pressure mercury lamp [13, 14]. The kinetic mechanism consists of the photolysis of the N 2 0 molecules following the scheme [ 13] N20 +hv (185 nm) ~ O('D) + N20

,20(3p)

and the subsequent oxidation of the silane. A different approach was tried with high energy photons provided by sources with emission spectra reaching further into the VUV in order to photodissociate mainly the silicon donor instead of the oxidant precursor. These incoherent VUV lamps use various gases such as deuterium (121.5 nm), argon (106.6 nm), xenon (147 and 129 nm) and krypton (123.6 nm). A krypton lamp was applied by Inoue et al. [22, 23] to photodissociate an Si2H6 and 02 mixture, obtaining an efficient excitation of Si2H6 to higher states (maximum absorption around 120 nm) yielding silicon hydride radicals in primary processes by the absorption of VUV light [34] as follows:

Si2 H6 O+O2+M

~O2+N2

O(ID) +N2 ,2NO

Si2H6 + Sill2-

Si2H6 + H Si2 H5 + Sill3

, Si3H*

, Si2H5 + H2 , Si3 H8

Higher silane intermediate products probably strongly absorb VUV light as well, and may therefore dissociate into additional radicals. In addition, oxygen is dissociated by the absorption of high energy photons between 133 nm and 200rim yielding excited (O(1D) and O(1S)) and ground state (O(3p)) oxygen atoms, following the scheme hv 3, O(3p)

0 2

+ O(IS)

~. < 133 nm

+ O(3p) + O(1D)

1 3 3 n m < 2 < 175nm

+ O(3p) + O(3p)

175 n m < 2 < 200 nm

hi,

02 hv

02

As explained before, ozone is also produced and, by UV radiation absorption, more O(~D) radicals can be obtained. The chemical reactivity of this radical is several orders of magnitude higher than those of O(3p) and O(~S), and therefore it is more effective for the production of SiO2 thin films. With VUV instead of UV radiation a more effective silicon donor precursor dissociation is achieved and thus different secondary processes are induced, leading to slightly changed film properties, such as reductions in the interface state density and the formation of Si-OH bonds [22, 23].

174

P. Gonzhlez et al. / Photo-CVD o f Si oxide

Although with a mixture of Si2H 6 and 02 silicon oxide can be thermally deposited at temperatures higher than 150 °C, under the VUV irradiation provided by xenon and D2 lamps [19, 20] the growth rate increases considerably, allowing the substrate temperature for obtaining SiO2 films to be decreased even to room temperature. Xenon lamps are more effective because the photoabsorption cross-section of 02 at 147 nm is larger than at 121 nm. For an Sill 4 and 02 mixture [ 18] the energy supplied to the system by a D2 lamp allows the substrate temperature threshold to be reduced to 80 °C, while in the case of thermal CVD the lowest temperature is 170°C. Moreover, for films deposited at the same temperature (175 °C) by thermal CVD and D2 lamp CVD, the optical and electrical properties obtained by photoCVD are better. In order to decrease the substrate temperature threshold even further this group has investigated more sophisticated systems based on the double photoexcitation of Si2H6 or Si3H8 and 02 using D2 and mercury or xenon lamps simultaneously [2729]. Details of these processes have not yet been clarified. Other oxidizing agents (N20, NO, N203) have been mixed with disilane [21] and irradiated with D 2 lamp photons in order to obtain silicon oxide films. The best optical and electrical characteristics have been obtained using NO and N203, since these gases are more effective than N20 in oxidizing the disilane molecules. In this line of high energy photon lamps, Marks and Robertson [24] have studied the process of SiO2 deposition from a gas mixture of SiH4 and NO2 using a windowless, microwave-excited argon lamp (106.6 nm). The use of an internal lamp obviates the need for a window between the lamp and the reaction chamber, thus eliminating attenuation of the VUV radiation by a window and the possible formation of deposits on it. The substrate temperature was selected between 25 °C and 250 °C and the films processed at high temperatures exhibited good electrical properties with a conformal step coverage and optical properties similar to oxides grown at 600 °C by thermal CVD. A similar study was carried out by Baker et al. [25] using an internal nitrogen lamp. As has been shown, with UV lamps significant reductions in the deposition temperature have been obtained, and in some cases the interface state density and OH content could also be lowered. Nevertheless, some drawbacks inherent to lamp processing should be mentioned, especially the low deposition rates due to low fluence. A very promising lamp technology, UV excimer radiation from dielectric barrier discharges [3540], at present in development, could overcome this drawback. The possibility of obtaining higher photon fluxes compared with conventional lamps, a larger

number of wavelengths and a variety of conceivable geometries makes these excimer lamps very attractive for many potential applications. One of these applications is the direct photo-induced deposition of SiO2 that was demonstrated by Bergonzo et al. [26] using Sill 4 and N20 which are photodissociated by the 126 nm photons generated by an Ar* excimer lamp. A full explanation of the processes involved in lamp CVD technology and an in-depth comparison of the results are not possible or, at best, will be very difficult, mainly because the process mechanisms are not sufficiently known and the results reported by different researchers are only partial. Yet more activity must be devoted to the scientific aim of bringing under one common general view all the lamp processing data.

3. Lasers

Other types of photon sources applied to photo-CVD processes are IR, visible and UV lasers owing to their monochromaticity, ability to be focused for localized deposition, and high energy density. In spite of their higher price, the major advantages of lasers compared with lamps are (1) photon fluxes that are orders of magnitude higher, (2) possibility for localized deposition, (3) spatial selectivity (perpendicular and/or parallel configuration), allowing separation of gas phase and surface processes that contribute to the film growth, and (4) energetic selectivity owing to the monochromaticity of laser photons. The type of laser radiation most widely used to induce silicon oxide film deposition (Table 2) is provided by the ArF excimer laser (193 nm) in both parallel and perpendicular configurations (Fig. 3). Several groups have studied the precursor mixture of S i l l 4 and N 2 0 [41, 43-48, 50-54, 56], adopting a parallel incidence of the laser beam. The irradiation of this gas mixture with ArF laser pulses (193 nm) produces photolytic dissociation of the precursor molecules. The single-photon absorption cross-section for silane is quite low: 1.2 x 10 -21 cm2 in the range 190-200nm [68]. More recently, silane dissociation by two-photon absorption has been experimentally demonstrated [69]. The photofragments resulting from the energetically lowest reaction channels are produced by the reactions [70] Sill4 + hv

' Sill2 + H2

Sill 4 + hv

, Sill 3 + H

Moreover, a large number of further reactions are possible such as disilane (Si2H6) and trisilane (Si3Hs) formation. These precursors also absorb UV photons and therefore participate in the process by increasing the coupled energy.

193 193

193 193 193

193

249 193

193 193

193 193 193

488

530.9

10.6 gm

10.6 lam

193

ArF ArF

ArF ArF ArF

ArF

KrF ArF

ArF ArF

ArF ArF ArF

Ar ÷

Kr +

CO 2

CO 2

ArF

Perpendicular, parallel

Parallel

Perpendicular

Perpendicular

Perpendicular

Perpendicular Perpendicular Perpendicular

Parallel Parallel+ perpendicular Perpendicular Perpendicular

Parallel

Parallel Parallel Parallel

Parallel Parallel

Configuration

q~ = 30 mJ cm -2 ~b = 30 mJ cm -2 -q~ = 38 mJ cm 2

P = 18-30 W

P = 12 W

P = 100 500 mW

I = 800-2100 W cm 2

4~ = 10 130 mJ cm 2 1 = 0.8 3.0 MW cm 2 4~ = 28 mJ cm 2

P = 0.5 3 W I = 1.5 × 107Wcm -2 I = 1.2 × 106 W cm 2 ~b = 10 180 mJ cm -2 P = 2W

q~ = 80-3800 mJ cm 2

• = 50-130 mJ cm -2 P = 4W P = 1.5 W

I = 40 W cm 2

I, 4~ or P

02 + DES 02 + TEOS

N20 + H M D S

N 2 0 + TMS

N 2 0 + Sill 4 02 + SiH4(N2) Sill 4 + N 2 0 ( A r )

N 2 0 + Sill 4 02 + Sill 4 N 2 0 + SiHn(N2)

02 + TEOS N 2 0 + Si2H6(N2) N 2 0 + Sill 4

N 2 0 + Sill 4 Air + HMDS(He)

O 2 + SiH4(N2) N 2 0 + SiH4(N2)

50 mbar 4 mbar -100 mbar

100 300 Torr

3.8 Torr

0-100 mbar 0 100 mbar 54-56 mbar

300 700 mbar 45 Torr 20 80 Torr

5-40 mbar 1 Torr

20 4 - 6 Torr

5-80 Torr

40-200 Torr 20 Torr 2 5 Torr

N 2 0 + SiH4(N2) N 2 0 + SiH4(N2) NeO + Si2H6(Ar ) Sill 4 + N 2 0 ( A r )

6 - 8 Torr

Total pressure

N20 + SiH4(Ar ) N20 + SiH4(N2)

Precursors"

aHMDS, hexamethyldisilane; TEOS, tetraethoxysilane; TMS, tetramethylsilane; DES, diethylsilane. bESR, electron spin resonance; other acronyms as for Table 1.

Wavelength (nm)

Laser

5

10 50 20 200

40

100

10 35

5O

20-70

100

Pulse repetition rate (Hz)

TABLE 2. Processing conditions for the SiO 2 thin films deposited by laser chemical vapour deposition

45 21 1.2 1.8

200

Rods

2000 50 800

960 60

300 900

420

200 860 20

3000

Growth rate (A min 1)

-60 22 - 5 0 250 50 500

Room temperature Room temperature 250 400

100-1200

70 370 Room temperature 180-280 250 25 400

175 250 100-500

160-300

200 300 200 300

20 600

(of)

Substrate temperature

Ellipsometry, IR, SEM, etch rate Ellipsometry IR, C V, SEM

Abbe refractometry, X-ray, Raman Etch rate, profilometry

IR, C - V , ellipsometry IR, C V, ellipsometry Ellipsometry, IR, C - V , SEM

Ellipsometry, IR, SEM, C-V, AES, Auger, etch rate IR, XPS, ellipsometry Ellipsometry Ellipsometry, C V, IR, ESR Ellipsometry, IR, SEM, etch rate Etch rate, IR, C - V Ellipsometry, IR, C - V AES, XPS, etch rate Ellipsometry, IR AES

Characterization techniques b

[61]

[67]

[65, 66]

[64]

[63]

[60] [61,621

[59]

[57] [581

[45, 56]

[55]

[50 54]

[46, 47] [48] [49]

[41] [42 45]

References

tan

¢3

176

P. GonzMez et al. / Photo-CVD o f Si oxide

Las e r

beam

Windows

I

1

Laser beam

Gas

Colli°atiOn optics H i H Gas

ub

I

,

exhaust

ter

inlet

Fig. 3. Laser CVD experimental system. Two possible configurations: parallel and perpendicular incidence.

On the contrary, the N 2 0 molecules exhibit an absorption cross-section of about (0.8-1.0) x 10 ~9cm-2, which is nearly two orders of magnitude greater than that of silane (refs. 32, p. 223, 46 and 71). N 2 0 photodissociation occurs by a single-photon absorption process ( 1 8 5 - 2 3 0 n m ) following the scheme (refs. 13, 32, p. 223, and 71-73) N 2 0 + hv ~

N2 + O(JD)

N 2 0 + hv

~NO + N

A large group of intermediate species can be formed and secondary processes can contribute as reported before for UV lamps: O(ID) + N 2 O

~ N 2 + 02

O(IU) + S20

, 2NO

O --~ 0 2

) 0 3

These products can again photodissociate by photon absorption: 03+hv

, O2 + O ( ' D )

02 + hv

~ 20(3p)

Chemical reaction between the photo-dissociated species takes place more readily between atomic oxygen and silane molecules or radicals, i.e. O(~D) + Sill 4

, O H + Sill 3

followed by a long series of reactions, e.g. O H + Sill4

~ H 2 0 + Sill3

Sill3 + Sill3

, Sill 2 + Sill 4

The products can react further with oxygen to yield products such as SiOm H,, or more complex species that can react and/or decompose on the substrate surface to form the SiO2 film. The Colorado University group [42-45, 56] pioneered the demonstration of the viability of deposition of silicon oxide thin films with a laser in the parallel configuration and at low substrate temperatures (100450 °C). Although their parameter values seem not to be optimized, since they use a high laser pulse repetition rate, high flow rates, and large N20:SiH 4 ratios, they demonstrated that high deposition rates (3000A min -1) [43, 44, 56] and good conformal step coverage with an absence of thinning and cracks [42] can be achieved. They reported also on the electrical properties of these films, but did not perform a systematic study of the influence of the processing parameters. A second group [46, 47] focused on the theoretical analysis of the process based on experimental observations. Using the absorption coefficients of the reactant gases, they put forward a simple model of the deposition process based on calculations of the photo-generated molecules per unit volume. They obtained an expression for the deposition rate as a function of the N 2 0 partial pressure, laser intensity and molecular cross-sections. Another researcher [48] has adopted the same system and gas mixture. His research is focused on achieving high deposition rates (860 ,~ min-~) but, although some processing parameter dependences are presented, his study is not systematic either. More recently, our group [50-54] has deepened this study by using Sill4, N 2 0 and argon as purging gas.

177

P. Gonzdlez et al. / Photo-CVD o f Si oxide

ABSORBED LASER POWER (W) 1.0

0.0 i

500

I

5.0

2.0 l

I

.I

l

t

"E 400 E

SI-O stretching

I

< 300L,I

-

~200 I I--

C.D

100

i

0

I

i

100 E N E R G Y DENSITY

I

200 (md/crn2)

i

300

Fig. 4. Linear dependence of the film growth on the ArF laser energy obtained by different researchers; D, Tate et al. [46]; + , Sabin [48]; *, Gonzfilez et al. [50].

We have studied the dependence of the growth rate and optical properties on partial and total pressures, substrate temperature, laser beam-to-substrate distance and laser beam parameters. We have also studied the optical and electrical properties of silicon oxide thin films deposited on various substrates such as silicon and I I I - V compounds (GaAs and InP) [51] and have investigated the ability of these films to protect metallic substrates against corrosion at high temperatures in aggressive environments [54]. Some results obtained by our group agree with those presented by other workers, such as the importance of the laser beam-to-substrate surface distance (a lower growth rate being obtained as the distance is increased) [42, 48], the linear dependence of the growth rate on the laser energy density (Fig. 4), and a typical effect found in photo-CVD in silicon oxide, namely the gradual film oxidation effect, which occurs when the N20:SiH 4 ratio is increased (Fig. 5). The IR spectroscopic measurements of Fig. 5 show that the Si-O-Si stretching band shifts from low frequencies to values corresponding to stoichiometric films (1070 cm -~) as the N20:SiH 4 ratio is increased. A decrease in the hydrogen content detected by the reduction in the Si H bending vibration (870 cm -~) is observed accordingly. On the contrary, for other processing parameters such as the N20 partial pressure (Fig. 6), the results obtained by different researchers do not agree, although the photon source and the precursors were the same and the processing systems were similar. These facts reinforce our understanding that a general and straightforward explanation of the photo-CVD process is very difficult to provide. Since there are a very large number of variables, more controlled data must be made available so that the photo-CVD process can be better understood.

-.1

12'00

1600 Wavenumbers (cm-1)

800

Fig. 5. IR spectra of samples processed by ArF laser CVD [51] at different N20:SiH 4 ratios: spectrum a, 0.5; spectrum b, 1; spectrum c, 1.25; spectrum d, 2; spectrum e, 4. Ratios of Si O stretching to Si-H bending band areas are as follows: spectrum a, 5.1; spectrum b, 7.3, spectrum c, 6.8; spectrum d, 11.2; spectrum e, 18.3.

Another group has used SizH 6 and N 2 0 instead of using Sill 4 [49]. They presented an exhaustive characterization of the effects of changing the N20:Si2H 6 ratio, revealing a great difficulty in oxidizing the disilane. They found that N20:Si2H 6 ratios around 200 are necessary to obtain films with a refractive index of 1.46, although the IR spectra do not exhibit even then the typical peak positions of stoichiometric silica. This different behaviour may be due to the fact that disilane absorbs 193 nm radiation more readily than silane, and suggests that a competing silicon-yielding reaction must be suppressed in order for SiO2 to be obtained. This behaviour of the N20-Si2U6 system agrees with that observed with D2 lamps [21] and with an ArF laser in the perpendicular configuration [60-62]. Different excimer radiation ( K r F laser, 2 = 249 nm) was used to decompose a gas mixture of Sill4, 02 and N2, deposition rates of 300/k min- ~at low temperatures

178

P. Gonz(dez et al. / P h o t o - C V D oJ' Si oxide 500

",~ 400 < 300

.0 200 I-

2

0 0

5

10

15

L__

20

i

25

J

30

35

N20 partial pressure (Tort) Fig. 6. Different growth rate dependences on the N 2 0 partial pressure found by different researchers using an A r F laser C V D system: ± , Tate et al. [46]: *, Sabin [48]; i~, GonzS, lez et al. [50].

(250 °C) [55]. Nishino et al. studied the dependence of the laser power, O2:SiH4 ratio and substrate temperature, and found that the role of the radiation is simply to initiate the chemical reaction. From their results, these researchers claim that the following possible sequence can be considered. (1) The deposition does not take place thermally withouth parallel laser irradiation. (2) Precursor molecules are dissociated by laser irradiation. (3) The dissociated radicals reach the substrate surface and react with the adsorbed oxygen species on the surface to form SiO 2 (4) Once SiO2 is formed by laser irradiation, subsequent layers are formed thermally (with a catalytic effect) without the presence of the laser radiation. Nevertheless, no further study has clarified how the catalytic process continues. Concerning the perpendicular configuration of the laser beam, the Colorado University group has tried a mixed configuration of the incident beam by directing the parallel beam transmitted (80°/,0 by the chamber back perpendicularly onto the substrate. In comparison with samples processed only in the parallel configuration, they have observed an increase in deposition rate, densification of the films and a slight increase in refractive index values. The additional energy supplied reduces the presence of the bonded hydrogen as shown in the IR spectra, in which no Sill or SiOH features appear [56]. Our comparative study of SiO2 [57] deposition from Sill 4 and N 2 0 in the perpendicular configuration revealed that the films contain fewer water and hydroxyl groups and exhibit a better spatial confinement and improved optical properties compared with films deposited with TEOS and 02 using the same experimental system. These results have been again confirmed in a

very recent study performed by Rieger and Bachmann [61]. They also use Sill4 and N20, and present the dependence of the growth rate and refractive index on the substrate temperature, N20:SiH 4 ratio, total pressure, and pulse repetition rate. For these processing parameters their reported tendencies are similar to those obtained by us in the overlapping ranges. Nevertheless, some differences do exist. For the pulse frequency dependence, they did not find any saturation of the deposition rate. However, for the higher laser fluences and total pressure applied by us, saturation of the deposition rate was observed. Comparison of the results obtained by our group [50-54, 57] with the laser in the parallel and perpendicular configurations but for similar processing conditions shows that the films produced under perpendicular photon irradiation may be deposited even down to room temperature, while for the parallel configuration the temperature threshold lies at 160 °C. Furthermore, with the laser in the parallel configuration, a higher degree of oxidation is observed for the same NzO:SiH 4 ratio. This effect may be the result of photo-induced desorption processes. However, evidence has been obtained showing that for the same stoichiometry the films irradiated during the growth present a different structure corresponding to a higher energetic input [74]. When an SizH 6 and N 2 0 mixture is used [60, 61], very high Si2H6:N20 ratios are needed in order to obtain stoichiometric films. The same effect was reported for other systems such as an ArF laser in the parallel configuration and also for D2 lamps [21, 49]. Other gas mixtures studied under perpendicular ArF irradiation of the substrate contain organometallic compounds such as H M D S [58, 61], TMS [61], TEOS [59, 61] and DES [61]. G o o d adherent films were prepared at low temperatures but they contained high concentrations of oxygen, hydrogen and carbon, and presented poor spatial confinement when oxygen was added, owing to ozone formation in the gas phase, which possesses a long lifetime. Only for the TEOS and 02 mixture [59, 61] and substrate temperatures higher than 400 :'C is it possible to deposit films with properties close to those of thermal CVD layers. However, at these high temperatures the only advantage offered by laser CVD over conventional processes is the possibility of local deposition on selected areas, if it is required. Moreover, Klumpp et al. [63] have attempted the photothermal deposition of SiO2 using N 2 0 - S i H 4 and 02 Sill 4 mixtures by heating the substrate with an Ar + ion laser (,i = 488 nm). The results were, however, disappointing because of the high desorption rate of oxygen from the surface which was driven by the increased substrate temperature. Nevertheless, local deposition of SiO 2 from Sill 4 and N 2 0 using a focused Kr + laser (). = 530.9 rim) [64] has been obtained, giving rise to

P. Gonz(dez et al. / Photo-CVD o f Si oxide

silica rods 50-100 ~tm in diameter and deposition rates 0.5-2 ~tm s ' with laser powers of 100-500 mW and N20:SiH 4 ratios between 17 and 53. If the success of amorphous hydrogenated silicon deposition by CO2 lasers is kept in mind, it is quite surprising that there are only a few papers on silica deposition from an Sill 4 and N20 gas mixture with CO2 lasers (10.6 lam). This type of laser is very appropriate for industrial applications because of its economy, reliability, and ease of handling. In the perpendicular configuration [65, 66] with quartz substrates, a selective area is deposited by pyrolysis of the precursors on the laser-heated substrate. The dimension and shape of the deposited films varied with laser beam size, source gases and other factors. In the parallel configuration the reaction is thermally driven by the gas heating produced by the successive multiple-photon resonant absorption of the 10.61am radiation by the silane molecules. In this configuration our group has performed an in-depth study of the influence of the processing parameters on the film properties and it has been demonstrated that the composition and structure of these layers depend mainly on the P N 2 0 / P s i H 4 ratio in the gas mixture. We have also deposited dense silica effective as a protective layer against metal corrosion at high temperatures [67]. As a consequence of the systematic study performed by our group on silicon oxide thin film deposition by UV and IR lasers in the perpendicular and parallel configurations, we have found some common behaviour in the optical properties exhibited by the films deposited from the same precursor gas mixture of Sill 4, N20 and argon. As shown in Fig. 7 the refractive index measured by ellipsometry (2 = 633 nm) vs. the N20:SiH 4 ratio is the same for different series of experiments performed with the ArF laser in parallel and perpendicular configurations and with the CO2 laser in the parallel configuration. The refractive index decreases very rapidly from high values, corresponding to suboxides, to 1.46, corresponding to stoichiometric silica [75, 76], indicating that a similar gradual oxidation of the film takes place as the concentration of the oxidizing agent increases essentially independently of the excitation mechanism. This behaviour is corroborated by the IR spectroscopic measurements commented on previously. However, some interesting differences between the perpendicular and the parallel configurations are observed in the IR spectra by studying the Si O-Si stretching band position and its full width at half-maximum. With either the CO2 or the ArF laser in the parallel configuration, the peak positions are the same as a function of the N20:SiH 4 ratio, whereas with the excimer laser in the perpendicular configuration the values are different, thus giving information on the effect of the additional

179

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A B C D E

oa

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SERIES SERIES SERIES SERIES SERIES

u

0

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110

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310

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N20/SiH4 RATIO Fig. 7. Refractive index vs. N 2 0 : S i H 4 ratio of the following series: series A, CO 2 laser, parallel configuration, T = 250 °C, PT = 100 150 Tort, P = 29 W, beam substrate distance d = 4 mm; series B, A r F laser, parallel configuration, T = 2 5 0 ° C , f = 1 0 H z , 4= 1300 mJ cm -2, d = 1.6 m m , PSiH4 = 0.8 Torr; series C, A r F laser, parallel configuration, T = 2 5 0 °C, f = 10 Hz, ~b = 1 3 0 0 m J c m 2, d = 0.7 ram, PS~H4= 0.5 Torr; series D, A r F laser, parallel configuration, T = 2 5 0 ° C , f = I O H z , q ~ = 1 3 0 0 m J c m 2, d = 0 . 7 m m , PsiH4= 0.8 Tort; series E, A r F laser, perpendicular configuration, T=270°C,f=40Hz, ~ b = 1 0 4 m J c m 2, P N 2 o = 1 9 . 4 T o r r .

photon energy supplied to the substrate and film surface by the impinging photons [74]. All these data indicate that in this case the main parameter controlling the various deposition processes is the N20:SiH 4 ratio. In spite of the thorough study of the Sill 4 and N20 precursor system with different wavelengths and configurations, the e x situ characterization of the films (ellipsometry, Fourier transform IR spectroscopy, Rutherford backscattering, SEM and electrical measurements) that has been performed must be complemented by in situ diagnostic techniques giving information about the gas phase and surface species involved in the processes in order for a complete view of the reaction and growth mechanisms to be obtained.

4. Summary Silicon oxide thin films can be prepared successfully on different substrates by photo-CVD processes using several precursor gases, experimental systems, and photon sources such as UV lamps and lasers. Technologically, this method has been successful in obtaining stoichiometric films and films that exhibit good properties for opto- and microelectronics and metallurgical purposes. From the scientific point of view several common points have been found for the photo-CVD processes presented: (1) the reduction in the substrate temperature threshold for obtaining films in comparison with

180

P. Gonz6lez et al. / Photo-CVD of Si oxide

thermal processes and especially when the photon incidence is perpendicular; (2) the gradual film oxidation when the ratio of oxygen donors to silicon donors is increased; (3) the dominance of the influence of the precursor gas ratio over the other processing parameters and even over different photon sources. However, it has been pointed out that the agreement is not total and different researchers studying the same system have reported different behaviours, dependences and conclusions. Thus, in spite of the amount of work done in this field, a host of points remain without a comprehensive explanation and a general theory of the photo-CVD processes is not available. Deeper kinetic studies, in situ analysis, control of the interface phenomena and the study of the early stages of the deposition are needed to achieve this aim.

Acknowledgments This work was partially financed under CEE-DGXII Contracts SC1.0202.C (JR) and M A l E 0029 U K and CICYT project MAT 88-0733.

References 1 P. Dimitrou, Dielectric Layers in Semiconductors: Novel Technologies and Devices, Editions de Physique, Les Ulis, 1986, p. 349. 2 J. Y. Chen, R. C. Henderson, J. T. Hall and J. W. Peters, J. Eleetrochem. Soc., 131 (1984) 2146. 3 H. M. Kim, S. S. Tai, S. L. Groves and K. K. Schuegraf, Proc. Electrochem. Soc., 81 (1981) 258. 4 R. Ashokan, V. Gopal and K. C. Chhabra, Phys. Status Solidi A, 121 (1990) 533. 5 Y. K. Su, C. R. Huang and Y. C. Chou, Jpn. J. Appl. Phys., 28 (1989) 1664. 6 C. J. Huang, Y. K. Su and R. L. Leu, J. Appl. Phys., 69(1991) 2335. 7 Y. K. Su, C. J. Huang, R. L. Leu and F. M. Pan, Solid State Eleetron., 34 (199I) 107. 8 Y. Mishima, M. Hirose, Y. Osaka, K. Naganime, Y. Ashida, N. Kitagawa and K. Isogaya, Jpn. J. Appl. Phys., 22 (1983) 46. 9 Y. Mishima, M. Hirose, Y. Osaka and Y. Ashida, J. Appl. Phys., 55 (1984) 1234. 10 Y. Mishima, Y. Ashida, M. Hirose and Y. Osaka, Extended Abstr. 15th Con£ on Solid State Devices and Materials', Tokyo, 1983, p. 121. 11 J. I. Takahashi and M. Tabe, Jpn. J. Appl. Phys., 24 (1985) 274. 12 Y. Tarui, J. J. Hidaka and K. Aota, Jpn. J. Appl. Phys., 23 (1984) L827. 13 M. Petitjean, N. Proust, J. F. Chapeaublanc and J. Perrin, Appl. Surf. Sci., 46 (1990) 189. 14 M. Petitjean, N. Proust, J. F. Chapeaublanc and J. Perrin, Appl. Surf Sci., 54 (1992) 453. 15 F. Houzay, J. M. Moison and C. A. Sebenne, Appl. Phys. Lett., 58 (1991) 1071. 16 C. Licoppe, C. Meriadec, J. Flicstein, Y. I. Nissim, E. Petit and J. M. Moison, J. Phys. (Paris), Ser. IV, I (1991) C2 357.

17 C. Licoppe, C. Meriadec, J. Flicstein, Y. 1. Nissim and A. C. Papadopoulo, Appl. Phys. Lett., 59 (1991)43. 18 M. Okuyama, Y. Toyoda and Y. Hamakawa, Jpn. J. Appl. Phys., 23 (1984) L97. 19 K. Tamagawa, T. Hayashi and S. Komiya, Photon, Beam, and Plasma Stimulated Chemical Processes at SurJaces, Materials Research Society, Pittsburgh, PA, 1987, p. 265. 20 M. Nakamura, M. Okuyama and Y. Hamakawa, Jpn. J. Appl. Phys., 29 (1990) L687. 21 Y. K. Bhatnagar and W. I. Milne, Thin Solid Films, 168 (1989) 345. 22 K. lnoue, M. Okuyama and Y. Hamakawa, Jpn. J. Appl. Phys., 27 (1988) L2152. 23 K. lnoue, M. Okuyama and Y. Hamakawa, Appl. SmJ~ Sci., 41 42 (1989) 407. 24 J. Marks and R. E. Robertson, Appl. Phys. Lett., 52 (1988) 810. 25 S. D. Baker, W. I. Milne and P. A. Robertson, Appl. Phys. A, 46 (1988) 243. 26 P. Bergonzo, P. Patel, I. W. Boyd and U. Kogelschatz, Appl. Sur£ Sci., 54 (1992) 424. 27 K. lnoue, M. Michimori, M. Okuyama and Y. Hamakawa, Jpn. J. Appl. Phys., 26 (1987) 805. 28 M. Okuyama, N. Fujiki, K. Inoue and Y. Hamakawa, Jpn. J. Appl. Phys., 26 (1987) L908. 29 M. Okuyama, N. Fujiki, K. lnoue and Y. Hamakawa, Appl. SurJi Sci., 33 34 (1988) 427. 30 I. W. Boyd, Laser Processing o£ Thin Films and Microstructures, Springer, Heidelberg, 1987, p. 224. 31 C. J. Huang and Y. K. Su, J. Electron. Mater., 19 (1990) 753. 32 H. Okabe, Photochemistry ~/" Small Molecules, Wiley, New York, 1978. 33 Y. I. Nissim, C. Licoppe, J. M. Moison, J. L. Regolini, D. Bensahel and G. Auvert, Proe. Soc. Photo-Opt. lnstrum. Eng., 1033 (1988) 273. 34 G. G. A. Perkins and F. W. Lampe, J. Am. Chem. Sot., 102 (1980) 3764. 35 B. Gellert and U. Kogelschatz, Appl. Phys. B, 52 (1991) 14. 36 B. Eliasson and U. Kogelschatz, Appl. Phys. B, 46 (1988) 299. 37 U. Kogelschatz, Appl. Surf. Sci., 54 (1992)410. 38 H. Kumagai and M. Obara, Jpn. J. Appl. Phys., 28(1989) L2228. 39 H. Kumagai and M. Obara, Appl. Phys. Lett., 54 (1989) 2619. 40 B. Eliasson and U. Kogelschatz, IEEE Trans. Plasma Sci., 19 (1991) 309. 41 H. Hasegawa, M. Akazawa, K. Matsuzaki, H. Ishii and H. Ohno, Jpn. J. Appl. Phys., 27 (1988) L2265. 42 P. K. Boyer, W. H. Ritchie and G. J. Collins, J. Electrochem. Soc., 129 (1982) 2155. 43 P. K. Boyer, G. A. Roche, W. H. Ritchie and G. J. Collins, Appl. Phys. Lett., 40 (1982) 716. 44 P. K. Boyer, C. A. Moore, R. Solanki, W. K. Ritchie and G. J. Collins, Extended Abstr. 15th Con/~ on Solid State Devices and Materials, Tokyo, 1983, p. 109. 45 K. A. Emery, L. R. Thompson, D. Bishop, H. Zarnani, P. K. Boyer, C. A. Moore, J. J. Rocca and G. J. Collins, Mater. Res. Soc. Symp. Proc., 29 (1984) 81. 46 A. Tate, K. Jinguji, T. Yamada and N. Takato, Appl. Phys. A, 38 (1985) 221. 47 A. Tate, K. Jinguji, T. Yamada and N. Takato, J. Appl. Phys., 59 (1986) 932. 48 E. W. Sabin, Proc. Electrochem. Sot., 86(1986) 284. 49 J. Shirafuji, S. Miyoshi and H. Aoki, Thin Solid Films, 157 (1988) 105. 50 P. Gonzfilez, T. Sz6rhnyi, M. D. Fern',indez, B. Ledn and M. Phrez-Amor, Surface Engineering with High Energy Beams, Trans Tech Publishers, Ziirich, 1990, p. 515.

P. Gonz(tlez et al./ Photo-CVD of Si oxide 51 P. Gonz'alez, E. Garcia, J. Pou, D. Fern~,ndez, B. Leon and M. P&ez-Amor, Appl. Surll Sci., 54 (1992) 108. 52 T. Sz6r6nyi, P. GonzS_lez, M. D. Fernandez, J. Pou, B. Leon and M. P~rez-Amor, Thin Solid Films, 193- 194 (1990) 619. 53 T. Sz6r6nyi, P. Gonz/dez, D. Fermindez, J. Pou, B. Le6n and M. Pbrez-Amor, Appl. Smf. &'i., 46 (1990) 206. 54 J. Pou, E. Garcia, P. Gonz~_lez, M. D. Fern&ndez, B. Le6n, S. R. J. Saunders and M. P6rez-Amor, Surfi Coat. Technol., submitted for publication. 55 S. Nishino, H. Honda and H. Matsunami, Jpn. J. Appl. Phys., 25 (1986) L87. 56 P. K. Boyer, K. A. Emery, H. Zarnani and G. J. Collins, Appl. Phys. Lett., 45 (1984) 979. 57 B. Le6n, A. Klumpp, M. P+rez-Amor and H. Sigmund, Appl. Surf Sci., 46 (1990) 210. 58 Y. Suzuki, Jpn. J. Appl. Phys., 29 (1990) L1517. 59 A. Klumpp and H. Sigmund, Appl. Surf Sei., 36 (1989) 141. 60 Y. Hiura, Y. Morishige and S. Kishida, J. Appl. Phys., 69 (1991) 1744. 61 D. Rieger and F. Bachmann, 2nd East- West Workshop on Laser

SurJaee Processing, Fonyod, Hungary, October 1991. 62 D. Rieger and F. Bachmann, Appl. Surf Sci., 54 (1992) 99. 63 A. Klumpp, C. Stumpff and H. Sigmund, Photon, Beam and

181

Plasma Enhanced Processing, Editions de Physique, Les Ulis, 1987, p. 507. 64 S. Szikora, W. Kr/iuter and D. B/iuerle, Mater. Lett., 2(1984) 263. 65 A. Sugimura and M. Hanabusa, Jpn. J. Appl. Phys., 26([987) L56. 66 A. Sugimura, Y. Fukuda and M. Hanabusa, J. Appl. Phys., 62 (1987) 3222. 67 D. FernS_ndez, P. Gonzfilez, J. Pou, B. Le6n and M. P6rez-Amor, Appl. Surf Sei., 54 (1992) 112. 68 J. H. Clark and R. G. Anderson, Appl. Phys. Lett., 32 (1978) 46. 69 C. Fuchs, E. Boch, E. Fogarassy, B. Aka and P. Siffert, Laser and Particle-beam Chemical Processing jbr Microelectronies, Materials Research Society, Pittsburgh, PA, 1988, p. 36[. 70 H. Stafast, Appl. Phys. A, 45 (1988) 93. 71 H. S. Johnston and R. Graham Can. J. Chem., 52 (1974) 1415. 72 J. Zavelovich, M. Rothschild, W. Gornik and C. K. Rhodes, J. Chem. Phys., 74 (1981) 6787. 73 I. P. Herman, Chem. Rev., 89 (1989) [323. 74 P. Gonz'alez, D. Fern/mdez, J. Pou, E. Garcia, J. Serra, B. Le6n and M. P6rez-Amor, in preparation, 1992. 75 G. Lucovsky, M. J. Manitini, J. K. Srivastava and E. A. Irene, J. Vac. Sci. Teehnol. B, 5(1987) 530. 76 W. A. Pliskin and H. S. Lehman, J. Electrochem. Soc., 112 (1965) 1013.